With popularization of smartphones in recent years, data traffic has rapidly increased. According to the Korea Communications Commission, accelerated popularization of smartphones has caused mobile data traffic to triple in the last year. As the number of users for smartphones will further increase and application services thereof will be more activated, data traffic is expected to increase even more in the future. In particular, when machine-to-machine communication (or the Internet of Things), which may encompass communication between humans and things and communication between things beyond communication between humans, is activated as new mobile commerce applications, it is expected that traffic passing through base stations will increase explosively in an unmanageable way.
Recently, direct communication between devices, known as device-to-device (D2D) communication, has attracted attention as a technology to solve the above problem. D2D communication may be enabled in both licensed bands for mobile communication and unlicensed bands for wireless LANs.
In particular, D2D communication combined with mobile communication may increase the traffic capacity of base stations and decrease overload thereof. By use of D2D communication, electronic devices or user equipments (UEs) in the same cell or neighbor cells may establish a D2D link between them and exchange data through the D2D link without passing through the base station or evolved Node B (eNB), contributing to halving the number of links to traverse (from two to one).
In the case of researches about unlicensed bands, the main object is to identify communication demands occurring between humans, between humans and things, and between things, and handle traffic arising in local regions in a suitable way while preventing unnecessary waste of radio resources. Thus, current researches focus on schemes that enable many proximate devices to broadcast and receive information about services and content items in an efficient manner.
It is required to establish synchronization between devices to control D2D communication. A device may establish synchronization with another device by use of timing information received from synchronous eNB or through a GPS (Global Positioning System) receiving module. In this synchronization scheme, the device should have access to a synchronous eNB or GPS module. However, a network operator may not support a synchronous eNB depending upon the communication mode. In this case, the device may fail to establish synchronization because it cannot obtain timing information from the synchronous eNB. Additionally, when the device is placed in a GPS shadow area (e.g. between high-rise buildings, tunnel, indoor site of a building), it may fail to establish synchronization because it cannot obtain timing information from the GPS. In other words, the device may fail to initiate D2D communication when the connection to a synchronous eNB or GPS receiving module is not good.